Add Prometheus metric to count upload errors (#8788)

Add Prometheus metric to count upload errors (#8775)

Add SeaweedFS_upload_error_total counter labeled by HTTP status code,
so operators can alert on write/replication failures. Code "0" indicates
a transport error (no HTTP response received).

Also add an "Upload Errors" panel to the Grafana dashboard.

weed/stats/metrics.go

@@ -458,6 +458,13 @@ var (
 			Name:      "bucket_object_count",
 			Help:      "Current number of objects in each S3 bucket (logical count, deduplicated across replicas).",
 		}, []string{"bucket"})
+
+	UploadErrorCounter = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Namespace: Namespace,
+			Name:      "upload_error_total",
+			Help:      "Counter of upload errors by HTTP status code. Code 0 means transport error (no response received).",
+		}, []string{"code"})
 )
 
 func init() {
@@ -519,6 +526,8 @@ func init() {
 	Gather.MustRegister(S3BucketPhysicalSizeBytesGauge)
 	Gather.MustRegister(S3BucketObjectCountGauge)
 
+	Gather.MustRegister(UploadErrorCounter)
+
 	go bucketMetricTTLControl()
 }